Thousands of US citizens may have been targeted in a huge tax scam run from call centres in Mumbai, where hundreds of workers were allegedly trained to speak in American accents in order to steal tens of millions of dollars, Indian police have said.

About 700 people are being investigated over what is believed to have been one of the biggest such scams in India’s history, which involved workers posing as US tax officials, according to Paramvir Singh, the police commissioner of Thane.

“Seventy workers have been formally arrested and around 630 others are being investigated,” Singh said. “We expect that many more people will be arrested.”

On Tuesday night about 200 officers raided nine premises in India’s financial capital. Police believe the alleged scam was run from the call centres, where workers pretended to be officials from the Internal Revenue Service (IRS), the US tax authority.

Employees would allegedly tell American citizens that they had defaulted on tax payments and were facing prosecution by the IRS. “They would give an American name and a batch number and tell the [US] citizen that they owed the authorities $4,000, $5,000 or $10,000,” said Singh.

“They were instructed to stay on the phone and told that their homes would be raided by police within 30 minutes if they hung up. They made threats, they said: ‘You have to pay, otherwise you will lose your job, your money, your house.’”

After allegedly duping the victims into revealing their bank details they would then withdraw money from their accounts, police said. The victims were told to stay on the call and go to their nearest Target or Walmart store, where they would buy a prepaid cash card, load thousands of dollars on to it and then transfer the money to an American bank account.

Police have not revealed the amount of money that was stolen, or whether citizens from other countries had been targeted. But Singh said the call centres were running for more than a year and are estimated to have conned billions of rupees out of thousands of people.

“We’ve been getting calls all morning from American citizens, people saying: ‘I think I got one of these calls. I think my money was stolen,’” he said.

Police suspect the ringleaders had associates in America, where the payments were processed.

The alleged scam was discovered followed a tip-off to police, said Singh, who sent in an undercover call centre worker to investigate. “We had a mole go in to the call centres to verify. The best part is that they were actually recording all their calls. We have recovered 851 hard disks on which the calls were recorded, so we’re going through those now,” said Singh.

US authorities had not approached Thane police on Thursday, but were expected to do so soon.

Many foreign firms outsource work to offshore call centres in India, where low-wage workers handle a variety of services, from reading out train timetables to selling mobile phone plans. In recent years, firms have started moving call centres to other countries such as the Philippines because of a preference for American-style English.

Thousands of US citizens may have been targeted in a huge tax scam run from call centres in Mumbai, where hundreds of workers were allegedly trained to speak in American accents in order to steal tens of millions of dollars, Indian police have said.

About 700 people are being investigated over what is believed to have been one of the biggest such scams in India’s history, which involved workers posing as US tax officials, according to Paramvir Singh, the police commissioner of Thane.

“Seventy workers have been formally arrested and around 630 others are being investigated,” Singh said. “We expect that many more people will be arrested.”

On Tuesday night about 200 officers raided nine premises in India’s financial capital. Police believe the alleged scam was run from the call centres, where workers pretended to be officials from the Internal Revenue Service (IRS), the US tax authority.

Employees would allegedly tell American citizens that they had defaulted on tax payments and were facing prosecution by the IRS. “They would give an American name and a batch number and tell the [US] citizen that they owed the authorities $4,000, $5,000 or $10,000,” said Singh.

“They were instructed to stay on the phone and told that their homes would be raided by police within 30 minutes if they hung up. They made threats, they said: ‘You have to pay, otherwise you will lose your job, your money, your house.’”

After allegedly duping the victims into revealing their bank details they would then withdraw money from their accounts, police said. The victims were told to stay on the call and go to their nearest Target or Walmart store, where they would buy a prepaid cash card, load thousands of dollars on to it and then transfer the money to an American bank account.

Police have not revealed the amount of money that was stolen, or whether citizens from other countries had been targeted. But Singh said the call centres were running for more than a year and are estimated to have conned billions of rupees out of thousands of people.

“We’ve been getting calls all morning from American citizens, people saying: ‘I think I got one of these calls. I think my money was stolen,’” he said.

Police suspect the ringleaders had associates in America, where the payments were processed.

The alleged scam was discovered followed a tip-off to police, said Singh, who sent in an undercover call centre worker to investigate. “We had a mole go in to the call centres to verify. The best part is that they were actually recording all their calls. We have recovered 851 hard disks on which the calls were recorded, so we’re going through those now,” said Singh.

US authorities had not approached Thane police on Thursday, but were expected to do so soon.

Many foreign firms outsource work to offshore call centres in India, where low-wage workers handle a variety of services, from reading out train timetables to selling mobile phone plans. In recent years, firms have started moving call centres to other countries such as the Philippines because of a preference for American-style English.